Add a default value to the dynamics

[roskuttan]

Add a default value to the dynamics

[danzimmerman]

Related to #73 and UniversalRobots/Universal_Robots_ROS2_Gazebo_Simulation#19

The Open Dynamics Engine fmax parameter was set to the joint's effort limit in gazebo_ros_control here.

It appears to default to zero in gazebo_ros2_control, and although it can be set via a URDF <dynamics> tag's friction attribute, that's not obvious or intutitive (Took me a while to find #73 to figure it out.)

[roskuttan]

Hi @danzimmerman, I appreciate your explanation about the 'fmax' parameter and its implications in gazebo_ros2_control. Based on my experience, I suggest that we consider changing the default value of the 'fmax' parameter. This might help to prevent confusion and unintended errors, especially for newcomers in this field.

Alternatively, adding a clear warning in the documentation about the potential issues when the dynamics tags are not properly set could be beneficial. This could highlight the importance of setting these tags and the potential problems that might occur if they are omitted.

I believe these changes could significantly improve user experience and make it easier for beginners to understand and use gazebo_ros2_control effectively. Thanks again for your insightful explanation.